This invention relates generally to communication networks and more particularly to systems for qualifying telephone lines for data transmission.
As is known in the art, public switch telephone networks, i.e., so-called plain old telephone service (POTS) lines, were originally designed for voice communications which cover a limited frequency bandwidth (i.e., about 4 KHz). Today, it is desired to use the same POTS lines for data transmission. Data signals, however, generally have different frequency characteristics than voice signals. As a result, a POTS line that works well transmitting voice signals might not work well, or at all, for data signals. Telephone companies need to know which lines are suitable, i.e., qualify, and which lines are not suitable for data transmission. Telephone companies also need to know why particular lines are unable to support data transmissions and where such faults occur so they can determine whether the transmission line can be corrected.
The telephone network was originally designed for voice communication. Voice communication covers a limited frequency bandwidth. In some cases, telephone lines were optimized for signals in this frequency range. Even where the lines were not optimized for voice signals, there was no incentive to make the lines operate at other frequencies and often they did not.
Now, it is desired to use those same lines to carry data signals. The data signals generally have different frequency characteristics than the voice signals. As a result, a line that works very well transmitting voice signals might not work well or at all for data signals. Phone companies need to know which lines will work for data signals and use those lines for data.
Line Qualification is the overall ability to make statements about the quality of a subscriber loop as it relates to its ability to deliver voice communications (i.e. POTS), or data services. Disqualification is the ability to make a statement with a high degree of confidence that a subscriber loop will not support a data service without remedial actions. Pre-qualification is the ability to make a statement with a high degree of confidence that a subscriber loop will support a data service without remedial actions.
Telephone operating companies (TELCO""s) have two problems to solve in qualifying subscriber loops for delivery of data. The first problem is strategic. Telco""s are reluctant to deploy emerging technologies for the delivery of data (e.g., ISDN or ADSL) because there is uncertainty in their knowledge that sufficient of the subscriber loops are of high enough quality to make deployment economically successful. This discourages early adopters because there is significant risk in being first to deliver a technology that may not work in their access network. If Telco""s could be given a technology to take much of this risk out of initial deployment, they can secure market share and lead in the face of competition
The second problem is tactical and comes after a Telco has made a decision to deploy a particular technology. There is a need to qualify, either pro-actively or reactively, specific lines for service as that service is requested by subscribers or targeted by the Telco for delivery. For example, if a Telco is to market and deliver the new service, they would like to target those subscriber loops most likely to support the service out of the box and/or with a minimum of work. As another example, a Telco receiving a new service request from a subscriber desires information to either accept or reject that request for new service based on the condition of their line.
4TEL, a product sold by Teradyne, Inc., of Deerfield, Ill., USA, has been used in the past in support of line qualification for delivery of POTS. Techniques in 4TEL lend themselves to the accurate detection and location of conditions which impair both voice and FSK modems. Modern data transmission techniques (such as those used in V.34, V.90, ISDN, and ADSL) encode data in part by shifting the phase of the carrier frequency(s). As such, these technologies rely upon there being fixed end-to-end and differential transmission characteristics (e.g., phase and echo).
A telephone line is made up of a two wire pair, called Tip and Ring. Ordinarily, the Tip and Ring wires should have the same electrical properties. It is desirable for the lines to be balanced. In a balanced line, the resistance, capacitance and inductance of each wire are equal. Imbalances exist if capacitance, inductance, or resistance of one of the wires differ from the other.
A particularly difficult type of condition to identify on a telephone line using single point measurements is called a series resistive imbalance. A series resistive imbalance introduces a differential phase shift between the two wires of the loop. The cause of series resistance is likely due to non-cold welded wire wraps, IDC, or dry solder joints. The oxidation created at the junction of the failing connection causes the series resistance to be unstable, thus modifying the phase shift with time due to changes in current flowing through the junction, further oxidation of the junction, mechanical movement of the junction, and the like. Higher speed modems encode many bits into phase shifts on these carrier frequencies. Thus even minor instabilities of the series resistance cause reduced data throughput, errors, and retraining. With ISDN, the shifts in phase cause energy from one pulse to overlap into the synchronization signal or into the time occupied by another pulse, thus causing inter symbol distortion and/or loss of synchronization. As can be seen, there is quite general degradation of both analog and digital transmission methods, both being susceptible to minor instabilities in series resistance. Stable series resistance, even when values get very high can often be successfully compensated for by internal circuitry in analogue modems or at the U interface for ISDN.
It is important to detect series resistive imbalance because large imbalance values affect POTS by reducing loop current levels. It is possible that the imbalance might be so large, (2 kilo-ohms or more) that seizing a dial tone may not be possible, or the ringing current might not be sufficient to activate the bell circuitry in the telephone or modem. It is equally important to detect imbalance at values below 2 kilo-ohm when data transmission is concerned. Any series resistance and the noise that it causes in terms of phase shift have a detrimental effect on the data throughput that may be achieved on that subscriber loop.
A telephone company would like to pre-qualify a line for high data rate operation, such as ISDN and ADSL. Lines that have been pre-qualified can be leased at a higher price. Lines with imbalances would not be made available for these high data rate services.
In accordance with the present invention, a method is provided for qualifying a transmission line to propagate data signals. The method includes measuring phase imbalance in the transmission line from a terminating end of the line.
When the wires get out of balance, a human user of the telephone line might notice a degradation in performance in the form of audible noise or reduced voice quality. When the line is used for data transmission, imbalance can limit the data throughput at which the line can operate. However, we have recognized that it is the change of imbalance that has most significant effect on data transmission.
In accordance with another feature of the invention, a method is provided for qualifying a transmission line to propagate data signals. The method includes measuring imbalance in the transmission line from a terminating end of the line.
In accordance with another feature of the invention, a method is provided for qualifying a transmission line to propagate data signals. The method includes applying a voltage in common (i.e., a common mode voltage) to the transmission line; and, determining phase imbalance in the line in response to the applied common mode voltage. The phase imbalance being representative of the difference in phase between the phase of a signal produced in one of the legs in the transmission line and the applied voltage; and, the phase of a signal produced in the other one of the legs in the transmission line and the applied voltage.
In accordance with another feature of the invention, a method for analyzing a transmission line wherein a common mode voltage having a frequency changing over a range of frequencies is applied to a pair of wires of in a transmission line; measuring the phase or magnitude of the signals in each wire of the transmission line relative to the applied common mode voltage in response to the applied common mode voltage over the range of frequencies; determining phase imbalance in the pair of wires in response to the applied common mode voltage over the range of frequencies; detecting a peak in the determined phase imbalance over the range of frequencies; determining a frequency of the detected peak.
In accordance with another feature of the invention, a method is provided for qualifying a transmission line to propagate data signals. The method includes applying a common mode voltage having a frequency changing over a range of frequencies into the transmission line; determining phase imbalance in the transmission line in response to the applied common mode voltage over the range of frequencies; detecting a peak in the determined phase imbalance over the range of frequencies; determining a frequency of the detected peak; determining line qualification in accordance with the determined frequency.
In accordance with still another feature of the invention, a method is provided for automatically qualifying a plurality of twisted pair transmission lines. The method includes feeding signals from a controller to a switch connected to termination ends of the transmission lines, such switch being coupled to a measurement unit. Test signals from the measurement unit are coupled to the transmission lines through the switch selectively in accordance with control signals fed to the switch by the controller. In response to the test signals, the measurement unit isolates resistance imbalance between each of the wires in the selected transmission line. The controller, in response to the isolated resistance imbalance, determines the qualification of the selected transmission line for data signals.
In accordance with still another feature of the invention, a system is provided for automatically qualifying a plurality of transmission lines. The system includes a switch coupled to terminating ends of the plurality of transmission lines. A controller is provided for feeding signals to the switch. A measurement unit is coupled to the switch and the controller. The measurement unit is adapted to feed test signals from the measurement unit to a selected one of the transmission lines through the switch. One of the transmission lines is selected in accordance with a control signal fed to the switch by the controller. The measurement unit isolates resistance imbalance between each pair of wires in the selected transmission line in response to the test signals fed to such selected transmission line. The controller, in response to the isolated resistance imbalance, is adapted to determine the qualification of the selected one of the transmission lines for data signals.
In accordance with another feature of the invention, a method is provided for determining the type of imbalance on a transmission line having a pair of wires. The method includes: feeding a frequency varying signal to the pair of wires; determining the phase imbalance in the pair of wires in response to the applied common mode voltage over the range of frequencies; measuring a frequency at a peak in the determined phase imbalance for a selected paired transmission line; and comparing the determined frequency to a pair of reference frequencies expected with a phase balanced pair of wires to determine the type of imbalance between the wires.
In accordance with yet another feature of the invention, a method is provided for locating the position of an imbalance on a selected test line. The method includes: applying a common mode, frequency varying voltage to twisted pair transmission line; measuring the phase of the voltages on each wire of the twisted pair transmission line relative to the applied voltage; computing the admittance of the twisted pair at the varying frequencies; deriving the capacitance over a selected transmission line from its measured admittance at the varying frequencies; dividing the derived capacitance by the per-unit length capacitance to ground for the transmission line under test to produce a quotient; computing the distance of the imbalance from the produced quotient.
In accordance with still another feature of the invention, a method is provided for locating the magnitude and position of an imbalance on a selected test line. The method includes: determining the presence of a series resistive imbalance; and if present, establishing the location and/or magnitude of the imbalance. The position of the imbalance is located by: applying a frequency varying, common mode voltage to the transmission line; measuring the magnitude and phase of the voltages on each wire of the transmission line; determining phase imbalance in the twisted pair in response to the applied common mode voltage; detecting a peak in the determined phase imbalance; determining a frequency of the detected peak; comparing the absolute value of the magnitude of the measured voltages and the detected peaks to a list of reference data for a transmission line of the type under test; determining the location of the imbalance based on this comparison. The magnitude of the imbalance is determined by: applying a common mode voltage to the twisted pair transmission line; measuring the magnitude and phase of the voltages on each wire of the twisted pair transmission line; determining phase imbalance in the twisted pair in response to the applied common mode voltage; detecting a peak in the determined phase imbalance; determining a frequency of the detected peak; comparing the frequency of the detected peaks to a list of reference data for a transmission line of the type under test; and, estimating the magnitude of the imbalance based on this comparison.